Synchronizing of deflecting circuits



Oct. 26, 1943.. K. wEND'r 24,332,681

SYNCHRONIZINGOF DFLECTING `GIRCUITS Filed Nov. 29, 1941 v Il. A Y ""Ilp |-Illll I U V41.

VDEO HMP Patented Oct. .'26, 1943 SYNCHRONIZING 0F DEFLECTING CIRCUITS Karl R. Wendt, Audubon, N. li., assguor to Radio Corporation of America, a corporation of Dela- Waffe Application November 29, 1941, Serial No. @23,924

6 Claims. (Cl. 17g-7.5)

My invention relate to the synchronization of cathode ray defiecting circuits, particularly as employed in television receivers.' I

An object of the invention is to provide an improved synchronizing system utilizing automatic frequency control in a television receiver or the like.

Another object of the invention is to provide an improved method of and means for synchronizing a cathode ray deflecting' circuit in accordance with the output of an automatic frequency control circuit.

A further object of the invention is to provide a television receiver synchronizing circuit that is not greatly affected by noise in the incoming signa1.

In one of the preferred embodiments of the in vention, a bridge circuit including two or more diodes is utilized for supplying a frequency control potential to a blocking oscillator or the like in the delecting circuit of a television receiver. A saw-tooth voltage wave may be applied across one bridge diagonal with the bridge circuit biased to prevent the passage of any diode current until a keying signal is applied to the other bridge diagonal.

In accordance with the present invention,` the keying pulses are taken from the intermediate frequency or i-f channel, preferably through a comparatively narrow pass band filter. The recurring peaks'of the modulated tf carrier which are representative of the incoming synchronizing pulses act las the keying pulses for the bridge circuit.

f It is assumed that the incoming signal is from a transmitter employing negative modulation and D. C. transmission, the modulating signal comquency control potential. Its amplitude varies v The invention will b e better understood from the following description taken in connection with the accompanying drawing in which the single figure is a block and circuit diagram of a television receiver embodying my invention.V

'I'he drawing shows a television receiver comprising the usual rst detector and tunable oscillator indicated at III, an i-f amplier I I, a second detector I2, a video amplifier I3, and a cathode ray tube Iii. Horizontal and vertical deecting coils I6 and Il, respectively, are supplied with saw-tooth current from the horizontal and vertical deflecting circuits for deflecting the cathode ray.

The deecting circuits may be of any suitable design. As illustrated for the horizontal deflection circuit, an oscillator of the well-known blocking oscillator type indicated at I8 may be employed for supplying pulses to a discharge tube (not shown) in a suitable saw-tooth wave circuit I9.

The oscillator I3 comprises a vacuum tube 2l,

a feedback transformer 22, va grid condenser 23, and a grid leak resistor 2| which is direct current connected to the plate of the preceding amplifier 26.

The horizontal deflecting circuit is held in synchronism with the corresponding deflection at the transmitter by means of the AFCvvoltage which is applied to the oscillator tube grid through amplifier 26. The AFC voltage is` ob. vtained as follows:

The i-f signal, which includes both picture and synchronizing signals, is supplied to an i-f amplifier 28 which preferably hasa narrow passA band as compared with the amplifier II. The output of amplifier 28 is applied through a coupling condenser 30 across one diagonal of a bridge circuit 29 which may include a pair of diodes 3| and 32 which are in two arms of the bridge and a center-tapped transformer winding 33. the two halves of which are in the other two arms of the bridge. I

The diodes 3| and 32 are connected cathode to plate through biasing resistors 34 ,and 38 which are shunted by condensers 31 and 38, re-

spectiv'ely.

The -f signal fromamplifler 28 is impressed across the bridge diagonal A-B through the transformer winding 33 which functions as an auto-transformer, the primary circuit being from the coupling condenser' 30 through a section of winding 33 to the center tap and through a conductor 4I and resistor I2 to ground.

'A saw-tooth voltage wave from the saw-tooth wave circuit I9 is supplied through a conductor I3 and a coupling condenser Il to the midpoint `of transformer 33. Thus, the saw-tooth wave is width or the like' of the i-f ampliier Il.

'to ground through a lter or storage condenser 46 to complete the circuit.

The voltage across condenser 46 is the frequency control voltage. It is applied through a D. C. connection to the grid of amplifier tube 26 whereby thek bias voltage on the control grid of the blocking oscillator tube 2| will depend upon the charge of condenser 46. Since the oscillator frequency depends upon the bias on its control grid its frequency will depend upon the value of the AFC voltage.

An RC illter comprising a large capacity condenser 4l and a high resistance resistor 48 may be connected in shunt to the condenser 46 to minimize the effect of any noise pulses that may pass through the bridge circuit and to stabilize the feedback circuit as described and claimed in my copending application Serial No. 404,787, med Juiy 31, 1941. l

In operation the pulses of i-f signal representing the horizontal synchronizingpulses, for example, cause current ow through diodes 3l and 32 and a bias voltage is built up across resistors 34 and 36 due to the slow time constant of these resistors and their shunting condensers 31 and their shunting condensers.

It will be apparent that my improved circuit makes the use of a synchronizing pulse separating circuit unnecessary and therefore simplifies the design. There is no -f signal appearing across the Vstorage condenser 46 because it is liltered out by the biasing resistors 34-36 and The bridge circuit need not be the lspecific one illustrated. For example, it may be of the type having a diode in each. bridge arm as described in my Patent 2,250,284, issued Julyl 22, 1941.

The vertical deflecting circuit may be synchronized in any suitable manner. For example, a circuit like that described above for horizontal deection may be employed with the i-f amplier .28 common to the two deecting circuits.

38. This bias prevents current flow through the diodes except during the occurrenceof the i-f synchronizing pulses. It will be noted that the bridge diodes also act as rectiers, only-the positive half cycle of the -f signal causing ailow of diode current.

Thebridgeisbalancedso that the synchronizing pulses do not appear across the diagonal C-D.

They act merely as keying pulses to permit the sawtooth voltage to pass through the bridge during their occurrence.

The time relation between the sawtooth wave and the keying pulses is such that the bridge is keyed "open to pass sawtooth voltage during the steepl portion of the sawtooth. The time relation between the vkeying pulse and the sawtooth wave determines the amplitude of the charging voltage pulse applied to the storage condenser 46 and, therefore, the magnitude of thel control voltage applied to the oscillator I6. In this respect the 1 operation is similar to that of my invention described and claimed in my copending application serial No. 404,787.

Since the diodes 3l and 32 function as peak rectiilers, the voltage appearing across a diode biasing resistor may be utilized for automatic volume control. `In the circuit illustrated the voltage across biasing resistor 36 is applied through a conductor 50 and through a smoothing lter comprising a resistor 5| and acondenser 52 to the I. F. ampliier Il.

An important feature of the invention is that 'the deilecting circuit is made less susceptible to noise in the incoming signal by reducing the'V band .width of the 'channel vsupplying signal to the bridge circuit. Thei-f ampliiier 261s given a comparatively narrow *band width, a width of one-half megacycle, for example, as compared with the fom` and one-half megacycle band The amplifier 26'will pass all the synchronizing pulse 'information that is required but, at the same time,- it will ilter out a large part of the noise passed by the I. F. amplier Il. The factthat much of the picture signal is also ltered out in this channel is immaterial. Since the ltering4 out of noise signals precedes the amplitude limiting action provided by the bridge circuit.

In the example illustrated, advantage is taken of the fact that separated vertical synchronizing pulses appear across the diode biasing resistors 34 and 36. Thus, these pulses may be taken off resistor 34 as shown, amplied by an ampliiierv condensers to provide a slower time constantv whereby the bias will hold over'between successive vertical synchronizing pulses.

. Saw-tooth voltage from the vertical deilecting sawtooth circuit 62 is supplied through a conductor 63 tothe other diagonal of the bridge 51 whereby the storage condenser 64 receives a charge which depends upon the time relation of the vertical synchronizing and the saw-tooth voltage wave. A lter 66-61 may be employed for the same purpose as Ethe filter 41'-46 in the horizontal deecting circuit.

I claim as my inventionz:

1. In a Atelevision receiver, a cathode ray deilection system comprising an oscillator, a balanced bridge circuit which may be keyed by sig-4 nal applied across onel diagonal to pass signal applied across Vthe other diagonal, means for applying across said one diagonal a carrier wave modulated by picture signal and synchronizing pulses with the carrier'wave of maximum amplitude representing said pulses, means for applying voltage waves obtained from said oscillator across said other diagonal, means for integrating the resulting output pulses of said bridge to obtain a frequency control voltage, and means for controlling the frequency of said oscillator in accordance with said control voltage. f

' 2. In a television receiver of the superheterodyne type having an intermediate ampliiier, `a cathode ray deilection system comprising an oscillator, a balanced bridge circuit which may be keyed by signal across one diagonal to pass signal applied across the other' diagonal, means for applying an intermediate `frequency signal representative of synchronizing pulses across said one diagonal to key said bridge circuit, means for obtaining pulses from said oscillator. each of which has a steep sloping edge, and applying them across said other diagonal with the steep of' said bridge to obtain a frequency control voltage, and means forA controlling the frequency of said oscillator in accordance with said control voltage. Y l A y 3. The invention according to claim 2 wherein said i-f signal is applied to said bridge circuit through a channel having a pass band that is `narrow compared with the pass band of said i-J amplier.

v4.1i television receiver of the superheterodyne type which includes a second detector coupled 'to an intermediate frequency amplifier that amv iiecting circuit which includes an oscillator, means for synchronizing said oscillator with said synchronizing pulses, said means comprising a bridge circuit having rectifiers in at least two arms oi the bridge, means for obtaining from said deflecting circuit a voltage wave having a sloping side and for applying it across one diagonal ot the bridge. means for so biasing said rectiers that said voltage wave will not causecurrent ow through them unless a keying pulse is being aP- plied to the other diagonal of the bridge, means for applying said modulated -f carrier wave to said other diagonal of the bridge whereby the bridge is keyed open to permit pulses of current therethrough which have an amplitude depend ing upon the time relation oi said synchronizing pulses and the sloping sides oi said voltage waves, means for integrating said current pulses to obtain a control voltage, and means for applying said control voltage to said oscillator to control its frequency.

5. The invention according to claim 4 wherein said modulated i-f carrier wave is applied to said other bridge diagonal through a channel having band o! said i-f amplifier whereby the eect of incoming noise signals on the deiiecting circuit is minimized.

6. A television receiver oi the superheterodyne type which includes a. second detector coupled to an intermediate frequency ampliiier that amplifies an i-f carrier wave modulated by picture signals and recurring synchronizing pulses, said pulses being represented by a carrier wave amplitude greater than the maximum amplitude representing picture signals, a cathode ray deflecting circuit for horizontal deflection which includes an oscillator, meansfor synchronizing said oscillator v with said synchronizing` pulses, said means comprising a bridge circuit having rectiiiers in at least two arms of the bridge, means for obtaining from said deilecting circuit a voltage wave having a sloping side and for applying it acrossone diagonal of the bridge, means for so biasing said rectiiiers that said voltage wave will not cause current ilow through them unless a keying pulse is being applied to the other diagonal of the bridge, said biasing means comprising a resistorcapacitor circuit. connected between said rectiers at the junction point ofthe bridge arms formed thereby, means for applying said modulated i-f carrier wave to said other diagonal of the bridge whereby the bridge is keyed open to a narrowpass band as compared with-the` pass permit pulses of current therethrough which have an amplitude depending upon the time relation of said synchronizing pulses and the sloping sides of said voltage waves,means for integrating said current pulses to obtaina control voltage, means for applying said control voltage to said oscillator to control its frequency, and

means for taking vertical synchronizing pulses 0E a portion of said resistor-capacitor circuit.

KARL R. WENDT. 

